1. Field of the Invention
The present invention relates to a suspension apparatus for a vehicle (hereinafter referred to as a “vehicle suspension apparatus”, and more particularly to a vehicle suspension apparatus which can control stiffness of a stabilizer bar.
2. Description of the Related Art
A conventional vehicle suspension apparatus of such a type includes a stabilizer bar and an oil-hydraulic cylinder. The stabilizer bar is mounted to the vehicle body at its torsion bar portion extending in the lateral direction of the vehicle in such a manner that the stabilizer bar can rotate about the axis of the torsion bar portion. The stabilizer bar has left and right arm portions extending from the left and right ends of the torsion bar portion in the fore-aft direction of the vehicle, and is connected to left and right wheel support members at distal ends of the respective arm portions. The oil-hydraulic cylinder is interposed between one of the arm portions and the vehicle body. Extension and contraction of the oil-hydraulic cylinder is controlled so as to control the stiffness of the stabilizer bar. Such a conventional vehicle suspension apparatus is disclosed in Japanese Patent Application Laid-Open (kokai) No. H5-201229.
In the vehicle suspension apparatus disclosed in the publication, a stabilizer bar includes a torsion bar portion extending in the lateral direction of the vehicle, and left and right arm portions extending from the left and right ends of the torsion bar portion toward the rear of the vehicle. One arm portion is connected at its distal end to one strut (wheel support member) via a stabilizer link, and the other arm is connected at its distal end to the other strut via an oil-hydraulic cylinder. The oil-hydraulic cylinder is disposed away from the torsion bar portion of the stabilizer bar and in the vicinity of the corresponding strut.
In the vehicle suspension apparatus disclosed in the publication, when the left and right wheels move vertically in the same direction (in the same phase) with extension and contraction of the oil-hydraulic cylinder being prohibited, the two arm portions and the torsion bar portion rotate together about the axis of the torsion bar portion. Therefore, in this case, the torsion bar portion is not twisted, and the stiffness of the stabilizer bar is maintained at a low level. When the left and right wheels move vertically in opposite directions (in opposite phases) with extension and contraction of the oil-hydraulic cylinder being prohibited, the two arm portions rotate in opposite directions about the axis of the torsion bar portion. Therefore, in this case, the torsion bar portion is twisted, and the stiffness of the stabilizer bar is changed to a high level.
Meanwhile, when the left and right wheels move vertically in the same phase with extension and contraction of the oil-hydraulic cylinder being permitted, as in the case where extension and contraction of the oil-hydraulic cylinder are prohibited, the torsion bar portion is not twisted, and the stiffness of the stabilizer bar is maintained at the low level. When the left and right wheels move vertically in opposite phases with extension and contraction of the oil-hydraulic cylinder being permitted, unlike the case where extension and contraction of the oil-hydraulic cylinder are prohibited, the torsion bar portion is not twisted, because the oil-hydraulic cylinder is extendable and contactable, and the stiffness of the stabilizer bar is maintained at the low level.
Incidentally, in the vehicle suspension apparatus disclosed in the publication, of the two arm portions of the stabilizer bar, one arm portion moves to follow the corresponding strut and the oil-hydraulic cylinder. Therefore, when the left and right wheels move vertically, input load always acts on the oil-hydraulic cylinder in the direction of extension and contraction, which brings about a problem of difficulty in securing durability of the oil-hydraulic cylinder.